(a) Field of the Invention
This invention generally relates to a method of fabricating a composite structure. More specifically, the invention relates to a method of fabricating a composite laminate comprised of a plurality of plies of fibrous material in an uncured resin matrix wherein voids in the laminate are substantially eliminated by expelling air entrapped in the laminate without the use of an autoclave.
(b) Description of the Prior Art
The present invention is concerned with fiber reinforced composite materials. As is well known to those versed in the art, a fiber reinforced composite normally consists of a resinous matrix sheet reinforced with a plurality of fibers embedded in the matrix. In some cases the reinforcing fibers have a random orientation. In other cases, the reinforcing fibers may have a parallel orientation and may be arranged in a laminate of several layers or plies, each having a parallel fiber orientation, with the fibers in the adjacent layers oriented in the same or different directions to provide a fiber reinforced composite with selected mechanical and thermal expansion properties.
Fiber reinforced composite may comprise various reinforcing fibers and various resins. Fiberglass, Kevlar, boron, and carbon are typical fiber materials. Epoxy and polyimides are typical resins.
As initially formed, these composite sheet materials are flexible and deformable, providing panel-forming members which can be draped or otherwise conformed to various shapes and thereafter cured, by the application of heat and pressure thereto, to tough lightweight, strong panels of permanent shape retention having high performance strength and stiffness characteristics imparted by the reinforcing fibers.
In the curing step, heat applied converts the liquid resin to a solid which bonds the reinforcing fibers into a rigid mass. The curing pressure is necessary to prevent shifting of the fibers during curing and thereby avoid formation of wrinkles in the finished molded article, to compress and expel entrapped air and excess resin from the composite material, and to consolidate the plies to a required thickness.
Typically, curing of fiber reinforced composite laminates has required the use of an expensive autoclave to provide high positive pressure at elevated temperatures. The high pressure has been necessary to remove and compress air entrapped between plies of the composite laminate during the lay-up operation. The air entrapped in the laminate tends to collect between the plies and, unless removed, significantly reduces the interlaminar shear strength and other matrix or resin critical mechanical properties. The application of high positive pressure compresses the laminate and physically forces air therefrom during the cure cycle. However, such high positive pressure also compresses the entrapped air. As such, some of the air may be retained in the laminate. This results in voids in the cured structure. However, if the positive pressure is sufficiently high, e.g., 85-100 psi, void content is significantly reduced, and a satisfactory part results.
While composite parts formed using an autoclave have been satisfactory from a structural standpoint, it has long been desired to replace the use of an autoclave with a vacuum furnace, i.e., an oven and vacuum source. This is basically due to (1) the high cost of autoclaves as compared to conventional ovens and vacuum pumps, (2) high tooling costs, both initial and maintenance, which would be significantly reduced because of both simplification and a decreased severity of the curing conditions, (3) component rejection rates, which would be minimized due to a decrease in the probability of vacuum bag loss by virtue of the reduced pressure differential employed, and (4) production rates, which would be increased due to availability of oven/vacuum pumps relative to autoclaves.
Attempts at vacuum curing composite laminates have not been successful because even in high vacuum, such as 5.times.10.sup.-4 mm, Hg., compression of the laminate has not been great enough to force sufficient removal of the entrapped air from within the laminate. As such, vacuum cured composite laminates have been highly porous with unsatisfactory mechanical properties.